In recent years, the importance of the digital coherent optical communication technology is increasing as the demand for faster networks with larger capacity grows. This communication method, also called intradyne reception, realizes a receiving sensitivity improved by 3 to 6 dB or more from On-Off Keying (OOK), Differential Quadrature Phase Shift Keying (DPSK), and other modulation methods that are widely used in conventional large-capacity optical communication systems. Other advantages of this method include the compatibility with polarization multiplexing method and multi-level modulation method such as Quadrature Amplitude Modulation (QAM).
FIG. 17 is a block diagram illustrating an example of a related digital coherent reception device (see Patent Literature 1 or Non Patent Literature 1, for example). Used as an input optical signal is a 4-channel (Ix, Qx, Ty, Qy) multiplexed signal which utilizes a polarization multiplexing (or Dual Polarization (DP))-Quadrature Phase Shift Keying (QPSK) signal. The input optical signal of each of the channels is separately converted into an analog electric signal by an optical-to-electrical (OE) converter and then into a digital signal by an analog-to-digital (AD) converter (ADC) that performs sampling in synchronization with a reference sampling clock (CLK).
Conventional (non-digital) coherent reception methods are not capable of stable reception due to a frequency/phase offset and polarization fluctuations of local oscillator (LO) light. However, the recent advance in the development of electronic devices has made high-speed AD converters available for use, and a frequency/phase offset, which has been a problem in conventional coherent reception methods, can now be compensated, as well as polarization fluctuations, by performing digital signal processing (DSP) on a signal that has been converted into a digital signal. As a result, stable, high-precision coherent reception is achieved. In digital coherent reception, wavelength dispersion compensation and more sophisticated waveform equalization technologies can be performed in addition to the frequency/phase offset compensation and polarized wave fluctuation compensation described above.
In an AD converter (see Non Patent Literature 2, for example) used in the related digital coherent reception device, intervals of discrimination levels are disposed as equally as possible by calibration or the like. The AD converter can thus have linear transfer characteristics as those illustrated in FIG. 18, and accomplish AD conversion that is higher in resolution (ENOB: effective number of bits) and low in distortion.    Patent Literature 1: Japanese Unexamined Patent Application Publication (JP-A) No. 2008-205654    Non Patent Literature 1: Seb J. Savory, “Digital filters for coherent optical receivers”, Opt. Express, Vol. 16, No. 2, 804-817, 2008    Non Patent Literature 2: Peter Schvan, “A 24 GS/s 6b ADC in 90 nm CMOS”, ISSCC Dig. Tech. Papers, pp. 544-634, February 2008